Field of the Invention
This invention is concerned with measuring systems and, more particularly, with errors generated by an analog-to-digital measuring system.
To measure physical phenomena, such as pressure, temperature, light, etc., a transducer is used to sense the physical phenomena. The transducer then produces an electrical representation corresponding to the sensed phenomena. This may be a voltage, current, resistance or capacitance. An electronic measuring system then takes this information from the transducer and generates an output quantifying the characteristics of the particular phenomenon.
The basic requirement for obtaining accurate results is that the transducer and the measuring system introduce no additional electrical representations so that only the actual physical phenomenon is measured. However, in the past, this problem has only been partially solved and then only with the use of extremely sensitive and costly devices.
In viewing a typical system as shown in FIG. 1, prior art, a physical phenomenon, such as heat, is measured by a thermocouple transducer 10. It produces a voltage Vn between leads 12 and 14 which identifies the difference in temperature between wires 12 and 14. The voltage Vn is a function of the original temperature (T2) of the reference junction of the thermocouple 36, and a function of the heat sensed by the transducer (T1). Since the temperature (T2) of the reference junction of the thermocouple 36 is usually not known, an error voltage Ve is introduced. The voltage Vn is provided to a measuring system 28 which includes a differential amplifier 20. Amplifier 20 is grounded 22 to the measuring system 28 which is usually different than the ground 16 of the transducer. As a result, a common mode voltage Vcm is introduced. Vcm varies as a function of the frequency and time. In addition, differential amplifier 20 introduces an offset error voltage Vos 18 which represents an incremental input voltage into the input of amplifier 20 necessary to provide an exact zero output voltage from the amplifier.
Some of the error voltages which have been generated in the measuring system have been negated by an apparatus disclosed in a previous patent issued to Mr. Jesse Lipcon, U.S. Pat. No. 3,982,241, issued on Sept. 21, 1976 and assigned to the same assignee of this instant invention. However, the problems of defining an absolute reference voltage and eliminating the common mode error voltage, while partially satisfied, were not completely addressed. Moreover, other prior art devices have not addressed this problem in the manner to be disclosed. These prior art systems involve substantial complexity by requiring a number of precise electrical components added to the measuring system as well as introducing a number of additional circuits to compensate for the errors generated. As is apparent, the above solutions require more complex circuitry as well as significant additional expense.